A particle beam therapy is a method for treating a cancer in such a manner that charged particles, such as protons or carbon ions, are accelerated to about several hundreds mega electronvolts by use of a device, such as an accelerator, which are then radiated to a patient to thereby give a dose to a tumor in the body. At this time, for the tumor, it is important to form a dose distribution that is as close as possible to a dose distribution ordered by a doctor, namely, a target distribution. In many cases, the target distribution is such a distribution in which the dose in the tumor is uniform and the dose outside the tumor is as lower as possible than in the tumor.
In general, when the particle beam accelerated by the accelerator is radiated to an object (including a case of human body), a three-dimensional dose distribution in the object has a feature of having a dose maximum peak at one given point. The dose maximum peak is called as a Bragg peak. Further, when it has a dose maximum peak at one point in a three-dimensional space, the position of such a peak is defined as “irradiation position” by that particle beam. In order to three-dimensionally form the target distribution using the particle beam with such a peak structure described above, some kind of ingenuity is required.
As one method for forming the target distribution, there is a scanning irradiation method. In order to employ this method, firstly, a feature is used that, using electromagnets, etc., arbitrarily deflects the particle beam in two directions perpendicular to a Z-direction that is a traveling direction of the particle beam, namely, X and Y-directions. Further, it is required to have a feature that adjusts energy of the particles to thereby arbitrarily adjust in the Z-direction, the position at which the Bragg peak is formed. Generally, a particle beam generation-transportation system that performs transportation and interruption of the accelerated particle beam is provided with the accelerator for accelerating the particle beam, and the accelerator also has an energy adjusting function. Then, upon setting a plurality of irradiation positions (referred to also as spots) in the tumor, the particle beam is radiated using the above two features, serially to the respective irradiation positions. The balance between the doses to be individually given to the respective irradiation positions has been adjusted and determined beforehand, so that the target distribution is formed as the result of totaling the respective dose distributions given to the respective irradiation positions.
Generally, the time taken to deflect the radiation direction of the particle beam in an X-Y direction to thereby move it from a given irradiation position to the next irradiation position, is 1 ms or shorter, and the time taken to move the position of the Bragg peak in the Z-direction by changing energy, is about 100 ms. Thus, generally, the irradiation sequence for the respective irradiation positions is such that, firstly, the particle beam is scanned with a single energy in an X-Y direction to thereby radiate the beam to all irradiation positions corresponding to that energy, and thereafter, the energy is changed to the next one.
At the time of movement of the irradiation position in the Z-direction by changing the energy, the radiation of the particle beam has to be always suspended, namely, interrupted. Depending on how to scan in the X-Y direction, the scanning irradiation method is subdivided further to the following respective methods.
The method in which the particle beam is interrupted during movement from a given irradiation position to the next irradiation position, is called as a spot scanning method, or a discrete spot-scanning method (see, for example, Patent Document 1, Patent Document 2).
For example, this method is achieved in such a manner that a feature of measuring the dose radiated to each irradiation position is provided, so that, at the time of reaching a predetermined dose value that is to be radiated to the irradiation position, the particle beam is interrupted and then the particle beam is moved to the next irradiation position.
In the case where the particle beam is not interrupted during movement from a given irradiation position to the next irradiation position, it is subdivided further to two methods. One of the methods is a method in which a feature of measuring the dose radiated to each irradiation position is provided, so that, at the time the dose reaches a specified value, the beam is scanned to the next irradiation position without interruption. This is called as a raster scanning method. Because irradiation is carried out during scanning of the particle beam, it is so adjusted that the total of the dose distribution given during the beam staying at the irradiation position and thus not being scanned, and the dose distribution given during being scanned, becomes the target distribution.
The other method in the case where the particle beam is not interrupted during scanning from a given irradiation position to the next irradiation position, is a line scanning method. This method is a method in which scanning is constantly continued, so that the particle beam is radiated to an irradiation target in such a manner that the particle beam does not stay at each irradiation position. A function of keeping constant a beam intensity that is a dose given per unit time and a function capable of arbitrarily changing a scanning speed are provided, so that, around the irradiation position where a large dose is to be given, the particle beam is scanned at a low speed, whereas around the irradiation position where a small dose is to be given, the particle beam is scanned at a high speed. In this manner, a final total dose distribution is adjusted to become the target distribution, by scanning the particle beam while adjusting the scanning speed to be inversely proportional to the dose to be given to each irradiation position.
According to the respective scanning irradiation methods mentioned above, because various uncertainties exist in actual irradiation, there is a possibility that, although the target distribution must be obtained on a calculation basis, the dose distribution actually obtained is not matched to the target distribution. The uncertainties include, for example, instability in intensity and/or position of the particle beam, an error in patient fixed position, an error in patient CT data, a signal delay and/or a noise of a control device, and the like. It is thought that, due to influence by them, an actual dose distribution possibly becomes different from the calculated values. Further, in the case where a tumor exists, in particular, in a respiratory organ, such as a liver, a lung, or the like, because the position of the tumor, the state around the tumor, or the like, changes temporally due to breathing of the patient, so that it is difficult to give a dose as planned to the diseased site.
As a method to solve the above problem, there is a method that is called as “rescan” or “repaint” as well (see, for example, Patent Document 1, Patent Document 2). This method is a method in which radiation of the particle beam to each irradiation position is performed plural times in a divided manner. This method is based on an idea of totaling the dose distributions at the plural times to average the errors therein to thereby cause error reduction. The number of divided times is called as a number of rescan times. The irradiation sequence is such that, firstly, the particle beam is scanned with a given energy in the X-Y direction to thereby radiate the beam once to all irradiation positions corresponding to that energy. Thereafter, radiation is performed again to each irradiation position without changing the energy. This is repeated the number of rescan times, and, after irradiating the number of rescan times, the energy is changed to the next one. The number of rescan times may differ depending on the energy, or may be the same for all of the energies. Generally, as the number of rescan times is increased, the influence by the above errors is averaged to become smaller.